Ovalbumin (OVA) is one of the major allergens in patients with food allergy. In order to delineate the antigenic determinants on OVA relating to hen's egg allergy, we studied the binding activities of IgE, IgG and IgA antibodies in patients with allergy to hen's egg white to two different OVA preparations, i.e. physically or chemically denatured OVA and enzyme-digested OVA fragments. The results obtained are as follows:1. The binding activities of IgE antibodies to the enzyme-digested OVA fragments were different from those of IgG or IgA antibodies. In addition, whereas the binding activities of IgE and IgG antibodies to fragmented OVA were decreased, those of IgA antibodies were not decreased, compared with those to native OVA. These findings suggest that the specificities of IgE antibodies to OVA are different from those of IgG or IgA and that human IgA binding is preferentially associated with sequential protein structures. 2. Anti-OVA antibodies in sera from patients with hen's egg white were mainly associated with IgG1 and IgG4. The binding activities of IgG1 antibodies to denatured or fragmented OVA were different from those of IgG4. These findings suggest that the specificites of IgG1 antibodies to OVA are different from those of IgG4.3. IgE antibodies in sera from most patients with hen's egg allergy bind to the OVA fragment with molecular weight of 10,000 in Western blots.